There have been numerous attempts to provide emulsions of high molecular weight silicones as an alternative to handling such materials in bulk. Generally, there are two types of processes to prepare emulsion of high molecular weight silicones, either via emulsion polymerization of siloxane monomers or mechanical emulsification of pre-formed high molecular weight silicones. Very often, there is a need to incorporate other components in a silicone composition to affect the properties of the silicone. For example, other additive components such as fillers, catalysts, or crosslinkers may be of interest in a silicone composition to alter subsequent physical properties of the silicone. However, incorporation of many of these components in a silicone emulsion composition may be problematic depending on the selection of the additive component and the emulsion process (that is mechanical vs emulsion polymerization process). For example, if the desired silicone additive component interacts with water during the emulsification process it may be difficult or impossible to incorporate its addition into a silicone emulsion.
There are abundant examples in the silicone emulsion art incorporating a variety of crosslinking components with organopolysiloxanes to provide high molecular weight or elastomeric silicone compositions. However, there are few examples for the addition of boron compounds as crosslinking additives in a silicone emulsion.
Many boron crosslinked organopolysiloxanes, or borosiloxanes, are of particular interest because of their inherent dilatant properties. For example, dilatant boron crosslinked silicones are used in active protection systems (APS) wherein a fabric is coated with a boron crosslinked silicone. However, such processes require the boron crosslinked silicone be first dispersed or dissolved in a solvent. Water based emulsions of boron crosslinked silicones would be easier to use and environmentally more desirable in such processes. However, there are few examples of emulsions of boron crosslinked silicones or borosiloxanes. Presumably, the interaction of a boron crosslinker with water has limited this crosslinking chemistry in a water based composition. Thus, a need exists to provide emulsions of boron crosslinked silicones to avoid undesirable solvent based processes.
U.S. Pat. No. 4,863,985 teaches aqueous silicone emulsions cross-linkable into an elastomeric state. In particular, U.S. Pat. No. 4,863,985 discloses thixotropic silicone emulsions cross-linkable into elastomeric state upon removal of water therefrom, e.g., to fabricate elastomer seals for the construction industry, have a pH of from 4 to 8 and a solids content of at least 50%, and contain: (A) 100 parts of an oil-in-water emulsion of an α,ω-(dihydroxy) polydiorganosiloxane, and a stabilizing amount of at least one anionic or nonionic surface-active agent, or mixture thereof; (B) 1 to 15 parts of a siliceous reinforcing filler in powder form; (C) 0 to 250 parts of an inorganic filler other than the siliceous filler (B); (D) 0.01 to 2 parts of a catalytic tin compound; and (E) 0.1 to 5 parts of boric oxide, a boric acid or a borate. While U.S. Pat. No. 4,863,985 teaches emulsions of silicones that provide elastomeric silicones, the addition of tin and fillers are necessary in these compositions. Furthermore, U.S. Pat. No. 4,863,985 teaches the addition of a boron compound to a pre-formed oil-in-water emulsion of α,ω-(dihydroxy) polydiorganosiloxane.
The present inventors have discovered a process for preparing emulsions of boron crosslinked organopolysiloxanes. In contrast to prior art teachings, the present inventors have unexpectedly discovered that boron compounds may be incorporated with an organopolysiloxane prior to forming an emulsion. The process provides emulsions of boron crosslinked organopolysiloxanes that yield subsequent silicone polymers of varying physical properties. In particular, the present process provides emulsions of boron crosslinked organopolysiloxanes that yield coatings that vary from a highly viscous silicone liquid to an elastomeric silicone having dilatant properties.